I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved system and method of deskew buffering signals in multiple rate communication systems.
II. Description of the Related Art
A mobile receiver in a portable communication system operates in an environment that subjects the receive signal to numerous degradations. The signal transmitted from a signal source is subject to numerous conditions, such as attenuation, interference, scattering, and reflections, prior to arrival at a receiver. The receiver must be able to recover the signal in spite of all these degradations in order for a successful communication link to be established.
Structures, such as buildings, and surrounding terrain, including walls and hillsides, contribute to the scattering and reflection of the transmitted signal. The scattering and reflection of the transmit signal results in multiple signal paths from the transmitter to the receiver. The contributors to the multiple signal paths change as the receiver moves. FIG. 1 shows a block diagram of a wireless communication system. A wireless telephone system is provided only as an exemplary embodiment.
In an exemplary embodiment, the wireless communication system may be a system such as a Code Division Multiple Access (CDMA) wireless system, consistent with xe2x80x9cTelecommunications Industry Association (TIA)/Electronics Industries Association (EIA)/IS-2000 STANDARDS FOR CDMA2000 SPREAD SPECTRUM SYSTEMSxe2x80x9d referred to as xe2x80x9cthe cdma2000 standard.xe2x80x9d In alternate embodiments, the system may be a system consistent with the xe2x80x9cTIA/EIA/IS-95 MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM,xe2x80x9d hereinafter referred to as xe2x80x9cthe IS-95 standard,xe2x80x9d or other systems such as described by xe2x80x9cANSI J-STD-015 DRAFT STANDARD FOR W-CDMA (WIDEBAND CODE DIVISION MULTIPLE ACCESS) AIR INTERFACE COMPATIBILITY STANDARD FOR 1.85 TO 1.99 GHz PCS APPLICATIONSxe2x80x9d referred to as xe2x80x9cW-CDMA,xe2x80x9d or other systems generally referred to as High Data Rate (HDR) systems.
A mobile phone 110 operating in a wireless communication system, such as an IS-95 system, uses radio waves to communicate with a base station 120. The base station 120 is identified by an antenna, although in reality the base station hardware would not be immediately located with the antenna. The base station 120 antenna may be located on a building 122 or may be located on an antenna tower. Although only one base station 120 is shown, the mobile phone 110 may simultaneously communicate with more than one base station 120. Transmissions from the base station 120 to the mobile phone 110 ideally traverse a single path, but in reality traverse multiple paths.
Terrain or a structure 130 may obstruct the signal path from the base station 120 to the mobile phone 110. The structure 130 that shadows the mobile phone 110 contributes to slow fade variations of the received signal power. Multiple signal paths from the base station 120 to the mobile station 110 occur because of reflections and scattering of the transmitted signal. Alternate signal paths may occur due to reflections off of structures 142, trees 144, and vehicles 146 that are sufficiently near the mobile phone 110. The objects that contribute to the multiple signal paths are centered about the mobile phone 110 in a radius that is proportional to the receive signal wavelength.
A mobile phone operating in a wideband system, such as a CDMA phone system, utilizes the signal bandwidth to its advantage when demodulating the received multipath signals. A coherence window is defined as the minimum time frame that can be discriminated. The coherence window is inversely proportional to the signal bandwidth. For a mobile phone operating in an Advanced Mobile Phone System (AMPS) that utilizes 30 KHz wide channel bandwidths, the coherence window is on the order of 1/(30 KHz)=30 uS. A mobile phone operating in a CDMA system that utilizes a 1.23 MHz channel bandwidth has a coherence window on the order of 1/(1.23 MHz)=800 nS. Thus, the mobile phone in an AMPS system can discriminate between multipath signals having a temporal spacing greater than 30 uS, while a mobile phone operating in a CDMA system can discriminate between multipath signals having a temporal spacing greater than 800 nS.
The ability to discriminate between multipath signals that are in close temporal spacing is used to improve signal quality. A CDMA receiver implements a plurality of demodulating fingers as a RAKE receiver. Each of the demodulating fingers is able to demodulate a multipath signal independent of the other fingers. The signals are then coherently combined in order to improve signal quality and reduce the effects of non-coherent noise. Since each finger tracks a different multipath signal, the demodulated signals at any instant are temporally offset from one another. The temporal offset must be compensated prior to coherent combining of the signals.
In order to temporally align all finger outputs, some type of deskew buffering configuration is required. What is needed is a deskew buffer configuration that enables efficient temporal alignment of the signals while using a minimum of resources and providing accurate signal buffering under all operating conditions.
The present embodiments disclose a novel and improved system and method of deskew buffering signals that minimizes or eliminates data loss at data rate change boundaries.
In one embodiment a demodulator finger is coupled to a deskew buffer. The demodulator finger receives a multipath signal and extracts a symbol from the signal. The demodulator finger has a PN counter that is used to generate a deskew buffer address. The deskew buffer address is modified by truncating a number of lower bits on the address and replacing the lower bits with a predetermined bit sequence. Alternatively, the number of lower bits may be logically AND""ed or OR""ed with a predetermined bit sequence. The symbol is written to the deskew buffer according to the modified deskew buffer address. The predetermined bit sequence is all ones in an embodiment and is all zeros in another embodiment. The number of lower bits that is truncated is related to a deskew index that corresponds to a data rate that is received. Each data rate uses a different Walsh length.
The embodiment may include a plurality of demodulator fingers. Each demodulator finger has a PN counter and is coupled to an independent deskew buffer. A combiner is coupled to all of the deskew buffers. A first demodulator finger is assigned to an earliest arriving multipath signal. Each of the additional demodulator fingers is assigned to one of a plurality of delayed multipath signals. Each demodulator finger demodulates symbols and uses their respective PN counters to generate a deskew buffer address. All of the deskew buffer addresses are modified according to the same deskew index and predetermined bit sequence. Each of the demodulator fingers writes the symbol in the deskew buffer location of their respective deskew buffers. The symbols are then read from each of the deskew buffers and summed in the combiner.
In an alternative embodiment, a plurality of demodulator fingers is coupled to a single deskew buffer. Each of the demodulator fingers has a PN counter and is assigned to one of a plurality of multipath signals. One of the plurality of demodulator fingers is assigned to an earliest arriving multipath signals. The demodulator finger assigned to the earliest arriving multipath signal demodulates the signal to extract the symbol. The demodulator finger uses the PN count to generate a deskew buffer address. The deskew buffer address is modified according to the deskew index and predetermined bit sequence as described above. The earliest arriving symbol is then written into the deskew buffer location determined by the modified deskew buffer address.
Each of the demodulator fingers that are not assigned to the earliest arriving multipath signal are assigned to one of a plurality of delayed multipath signals. Each of the plurality of demodulator fingers extracts a delayed symbol and uses their respective PN counters to generate a deskew buffer address. The deskew buffer address is modified in the same manner as was used for the earliest arriving symbol. Each demodulator finger, in the order of the time of arrival of the delayed multipath signals, reads the contents in the modified deskew buffer address, adds the delayed symbol, and writes the updated value back to the modified deskew buffer address.